Integrated electronic sensor for characterizing physical quantities and process for producing such a sensor

ABSTRACT

The integrated electronic sensor for characterizing physical quantities comprises a substrate (5) having a characterizing member (3) and a cover (9) maintained in the vicinity of and in front of the characterizing member (3) by means of at least one ball (11) bearing on the substrate (5), so as to provide a first spacing between the cover (9) and the substrate and is characterized in that the cover has a boss (15) facing the characterizing member (3) so as to define a second spacing between the cover (9) and the member, which is smaller than the spacing between the cover and the substrate.

DESCRIPTION

The present invention relates to an integrated electronic sensor forcharacterizing a physical quantity incorporating a characterizingmember, whereof one part is mobile.

The invention more particularly applies to the field ofmicroelectronics, where the development of micromachining methodspermits the production of characterizing members able to transformquantities such as acceleration, pressure or flow into characterizableelectrical effects by the measurement or acquisition of signals on a hitand miss basis.

Sensors of this type have a characterizing member mobile relative to asubstrate which, on deforming or moving, creates either an electricalpotential, or gives rise to the modification of the value of acapacitor, inductance or resistor. The modification of thesecharacteristics is exploited by an electronic reading circuit associatedwith the component. An illustration of the technologies involved insensors is given by the article "Low-cost integrated silicon sensor" byFouad Rahali et al., 9th European Hybrid Microelectronics Conference, pp243 to 249.

These sensors also have a cover, which has a number of functions. Itmust protect the characterizing member against environmental stressesand must permit its maintenance, particularly in the case of physicalstresses exceeding the mechanical limits of the member.

In an accelerometer incorporating a seismic mass able to move under theeffect of an acceleration, the cover serves as an abutment forpreventing an exaggerated displacement of the mass, which may give riseto the deterioration thereof.

Generally, a spacing is provided between the cover and the sensitivecharacterizing member so as to prevent, in normal operation, contactbetween said two parts. Thus, the cover does not impede the movement ofthe characterizing member and leads to no deterioration of the result ofthe measurements. Such an arrangement is described in U.S. Pat. No.5,164,328, where the cover is also formed by an integrated circuitsubstrate. It is known from the latter document to assemble the coverand the substrate having the characterizing member by means of aninterconnection procedure using meltable material balls and generallyknown as flip chip.

This procedure consists of depositing on one of the parts to beassembled, meltable material assembly rolls or balls and to raise thetwo parts to be assembled, which face one another, to a temperatureslightly above the melting point of the material of the balls, so as tobring about a type of brazing.

Preferably, on the parts to be assembled are previously formedinterconnection elements constituting a wetting surface with respect tothe balls or rolls. The elements respectively of each part to beassembled are positioned facing one another and the balls are onlydeposited on the elements of one of the parts.

The assembly of the cover and the substrate incorporating thecharacterizing member by the flip chip procedure is very interesting,because the interconnection balls ensuring the cohesion of the structurealso permit an electric contact when they are conductive. Anotheressential function of the balls is to maintain and define the spacingbetween the cover and the characterizing member.

Unfortunately, this solution is difficult to apply in numerous sensors.

Thus, due to a miniaturization of the sensors linked with the emergenceof new machining technologies on the surface (such as etching), thespacing between the cover and the sensitive characterizing member mustbe very small and in particular obtained with very high precision. Forexample, in the case of an accelerometer, there is a spacing of 2 to 5μm between the cover and the seismic mass.

However, with existing flip chip interconnection procedures, it is verydifficult to attain spacings below 15 μm with a sufficient accuracy.

Moreover, when the interconnection balls are of small size for reducingthe spacing between the cover and the characterizing member,thermomechanical behavioural problems occur.

As the expansion coefficients of the interconnected parts can differwidely, significant stresses may appear in the balls under the effect ofa temperature variation.

In the same way, when the substrate and cover are of the same nature andwith the cover generally incorporating a reading circuit, the latterheats in operation and consequently expansion differences exist betweenthe cover and the substrate.

The solution to this problem consisting of increasing the size of theballs would, in the present case, run counter to the sought aim ofreducing the spacing between the characterizing member and the cover.

Therefore the invention relates to an electronic sensor and to itsproduction process permitting a solution to the aforementioned problems.

An object of the invention is to provide a sensor, in which the distancebetween the cover and the characterizing member is both very small andcontrollable with a very high degree of precision.

Another object of the invention is to provide an interconnection betweenthe cover and the characterizing member, which has a good resistance tothermomechanical stresses and which can be very easily implemented.

Therefore the invention relates to an integrated electronic sensor forcharacterizing a physical quantity, incorporating a substrate having acharacterizing member, whereof part is able to deform in at least onedirection, and a cover maintained in the vicinity of and in front of thecharacterizing member by means of at least one meltable material ballintegral with the substrate and having a height adapted to thethermomechanical stresses of the sensor, so that a first spacing isprovided between the cover and the substrate, characterized in that thecover has a boss with a controlled thickness facing the characterizingmember, so as to define a second spacing between the cover and themember which is smaller than the spacing between the cover and thesubstrate.

This boss, whose thickness can be controlled in a simple and precisemanner, makes it possible to very accurately adjust the free distance ofthe cover up to the characterizing member.

The boss also serves to prevent an excessive displacement or deformationof the characterizing member or the mobile part thereof. To this end,the boss has an abutment surface against which the characterizing memberbears when subject to an excessive stress.

In general terms, the second spacing between the cover and thecharacterizing member is at the maximum equal to the travel authorizedprior to the breaking of the characterizing member.

The boss can also protect the mobile part of the characterizing memberby preventing it from performing a displacement in a direction notcorresponding to the intended displacement direction. For this purpose,the surface of the boss facing the characterizing member isadvantageously oriented parallel to the displacement direction of themobile part of the characterizing member.

At a minimum, the size of the boss will be that of the mobile part whichit faces. It could optionally be machined in order to comply withcertain geometrical criteria of said mobile part. In particular, theboss can be etched in accordance with the same geometry as the mobilepart (e.g. in comb form). It could be made from an insulating orconducting material if it was wished to use it as a second electrode ofan electrical circuit, the mobile part then forming the first electrode.

In the case of hit or miss characterizations, the boss is made from anelectrically conductive material. The boss may only have oneelectrically conductive material part, which then forms the abutmentsurface. The conductive material can e.g. be aluminium.

This particular case where the boss is conductive may also make itpossible to obtain a sensor sensitive in three directions, namely thedirections x and y parallel to the surface of the sensor substrate likethe conventional sensors described hereinbefore and direction zperpendicular to the sensor substrate surface, by electrical contactbetween the mobile part and the boss or by modifying the capacitancebetween these two conductors. Such a sensor can in particular be used inthe characterization of an acceleration in space and therefore in theproduction of a three-dimensional accelerometer.

The cover may simply be stuck to the balls, which maintain it at adistance from the member. However, advantageously, the cover and themember are connected to an interconnection process using meltablematerial balls using the flip chip procedure. These balls can also bemeltable material rolls.

As a result of the invention, the interconnection balls can be inaccordance with standard knowledge. Effectively, in the sensor accordingto the invention, the balls ensure the mechanical maintenance of thecover and possibly the electrical contact, but do not serve to definethe second spacing between the cover and the characterizing member. Thisspacing is defined by the thickness of the boss of the cover, which ispositioned facing the characterizing member.

However, the balls do make it possible to define the first spacingbetween the cover and the substrate.

Moreover, if the boss is sufficiently thick, the balls can be of largesize and consequently obviate the problem linked with mechanicalstresses associated with a possible difference between the expansioncoefficients. Larger balls also permit easier production.

According to another interesting feature of the invention, the cover isformed by a substrate having an electronic reading circuit.

The cover and the substrate are provided with interconnection elementsto which the characterizing member and the electronic reading circuitare respectively connected. These elements are used for the depositionof balls for interconnection purposes and with said balls ensure theelectrical connection between the characterizing member and theelectronic circuit.

The invention also relates to a process for producing a sensor.

Other features and advantages of the invention can be gathered from thefollowing illustrative and non-limitative description with reference tothe drawings, wherein show:

FIG. 1 A diagrammatic sectional view of a sensor according to theinvention.

FIGS. 2A & 2B Diagrammatic plan and sectional views along line A--A ofthe etched substrate incorporating the seismic mass of the sensoraccording to the invention.

FIG. 3 A sectional view of different stages of the production of asensor according to the invention.

As can be seen in FIG. 1 , the sensor 1 has a substrate 5 with acharacterizing member 3 obtained by micromachining. The substrate 5 isconnected to the cover 9 by means of interconnection balls 11 located oneither side of the characterizing member 3. These balls maintain thecover 9 at a distance H from the substrate 5.

The cover 9 is formed by a substrate 13, preferably of an insulating orsemi-insulating material and a boss 15 formed on the substrate 13 in theregion facing the characterizing member 3. This boss 15 makes itpossible to reduce and accurately define compared with the prior art thespacing h between the cover 9 and the characterizing member 3. Thisspacing h is much smaller than the spacing H between the substrate 5 ofthe characterizing member and the substrate 13 of the cover 9, i.e. inthe region surrounding the boss 15.

On their facing surfaces 18 and 17, in the region surrounding the boss15 and the characterizing member 3, the cover 9 and the substrate 5 ofthe member 3 are provided with interconnection elements 19, 19', whichpermit the attachment of the balls 11 and a good electrical contact forthe same. These balls are used for connecting the characterizing member3 to an electronic circuit 21 produced in the substrate 13 of the cover9, e.g. beneath the boss 15.

In order to avoid the aforementioned thermomechanical stress problems,the spacing H (height of the balls) is made adequate compared with thesize of the chip (substrate). Typically, a choice is made of H>10⁻² ×D,in which D is the size or the largest dimension of the chip (substrate).For example H≧45 μm for a substrate 5 of 4×4 mm. In this case,definition takes place of the boss 15 with a thickness of 40 μm so as tohave a spacing h between the cover and the characterizing member of 5μm.

Current leads 23 and 24 respectively formed on the substrate 13 and thesubstrate 5 connect the characterizing member 3 and the electroniccircuit 21 to the balls 11, via the elements 19, 19'. In this example,the leads 24 are integrated into the substrate 5.

The electronic circuit 21 is connected, via tracks 26 integrated intothe substrate 13, to external contact elements 25 formed on the coversurface 18. These elements 25 are themselves connected e.g. to tracks ofa printed circuit 27 on which the sensor is mounted via the cover.

In the case of an accelerometer, the characterizing member 3 canincorporate, as shown in FIGS. 2A and 2B, a seismic mass 29, which isparallelepipedic in the plane (x,y) of the substrate surface and whichis the plane of FIG. 2A.

The seismic mass 29 is connected to the substrate 5 by two flexiblebeams 31, 31' and is provided, in the direction opposite to that of thebeams, with teeth 33 extending freely into grooves 35 made in thesubstrate. The mass 29 is displaceable in the direction y parallel tothe substrate plane x, y.

Lateral portions 37, 39, in direction x, of the teeth 33 of the seismicmass 29 and grooves 35, are respectively covered with a metallic coatingand/or are themselves of a conductive nature, e.g. of doped silicon,forming the plates of a variable capacitance capacitor 41. Under theeffect of an acceleration, the seismic mass 29 is displaced and thethickness of an air layer 43 located in the interstices between theparts 37, 39 is modified. This air layer 43 constituting the dielectricof the capacitor means that the value of the capacitance measured on theplates is also modified.

FIG. 2B illustrates the movement of the seismic mass in direction y,level with the teeth 33.

It is necessary to ensure that an acceleration in direction zperpendicular to the substrate surface, i.e. to the plane x,y, does notgive rise to a movement of the seismic mass of greater amplitude thanthe depth of the grooves in which the teeth are located in order not todislodge the latter.

It must also protect the seismic mass against an excessive movementexceeding the yield strength of the beams 31, 31'. Thus, a stop must beprovided at a distance of the same order of magnitude as the depth ofthe grooves 35, i.e. 5 to 20 μm.

According to the invention, the movement along axis z is limited by theboss 15 provided with an abutment surface 16. For this purpose the boss15 must be at a spacing h from the surface of the substrate 5 ofpreferably 0.5 to 5 μm.

FIG. 3 illustrates in section a process for producing the sensoraccording to the invention.

Firstly one or preferably several interconnection elements 19' and 19are produced, respectively on the substrate 13 of the cover 9 and on thesubstrate 5 of the member 3 (parts a and f), e.g. by the vapourdeposition of a conductive material through a mask. This material mustbe wetting with respect to the material forming the balls 11.

The elements 19' are located on the face 18 of the cover 9 which will bebrought towards the substrate 5 having the characterizing member 3during assembly. The elements 19 of the substrate 5 are producedalongside the characterizing member 3 and on either side of the latter.

The elements 19, 19' will e.g. be of gold or silver and the balls 11 oftin and lead or indium alloy or any other meltable alloy.

The elements of the cover and the substrate are arranged in such a wayas to respectively face one another during assembly.

On the substrate 13 equipped with its interconnection elements 19' isdeposited an insulating or optionally semiconductor material coating 47(part b). As this coating is used for the formation of the boss, itsthickness must be carefully checked. Preference is given to aphotosensitive or non-photosensitive polyimide coating, whose thicknessin the range approximately 10 to 40 μm can easily be checked,particularly using the trammel deposition process.

After producing a mask 49 using photolithography procedures on thecoating 47 (part c), etching thereof takes place so as to define theboss 15 and expose the interconnection elements 19' (part d). Theetching of the polyimide coating 47 can take place by oxygen plasma orwet chemical etching when the polyimide is not photosensitive.

The mask is also eliminated and on the interconnection elements 19' areformed meltable material balls 11 (part e). These balls 11 are depositedby standard vapour deposition or electrolysis processes.

In another stage (part f), the substrate 5 is joined with the substrate13. This operation takes place at a temperature slightly above themelting point of the balls, so as to render integral the two substrates.

According to constructional variants, it is possible to join the coverto the substrate of the characterizing member or the substrate of thecharacterizing member to the cover. In the same way, the interconnectionballs can be produced either on the elements of the cover, or on thoseof the characterizing member substrate.

When the characterizing member is very sensitive, it may be preferableto produce the interconnection balls on the cover.

Finally, as a result of the invention, by combining an e.g. polyimideboss, whose thickness is accurately controlled, and the use ofconnection balls adapted to the size of the sensor substrate or chip, itis possible to obtain a spacing 4 calibrated to within 1 micrometerbetween the cover and the characterizing member, and an excellent sensorreliability.

I claim:
 1. Integrated electronic sensor for characterizing in aphysical quantity, incorporating a substrate (5) having a characterizingmember (3), wherein a portion of said characterizing member is able todeform in at least one direction (y,z), and a cover maintained in thevicinity of and in front of the characterizing member (3) by means of atleast one meltable material ball (11) integral with the substrate (5)and having a height adapted to the thermomechanical stresses of thesensor, so that a first spacing (H) is provided between the cover (9)and the substrate, characterized in that the cover (9) has a boss (15)with a controlled thickness facing the characterizing member (3), so asto define a second spacing (h) between the cover and the member (3)which is smaller than the spacing between the cover and the substrate.2. Sensor according to claim 1, characterized in that the boss (15) hasan abutment surface (16) for the characterizing member.
 3. Sensoraccording to claim 2, characterized in that the abutment surface (16) isparallel to said direction (y).
 4. Sensor according to claim 2,characterized in that the abutment surface (16) is substantiallyperpendicular to said direction (z).
 5. Sensor according to claim 4,characterized in that the boss (15) is made from an electricallyconductive material.
 6. Sensor according to claim 4, characterized inthat the boss (15) has an electrically conductive material part, whichforms the abutment surface.
 7. Sensor according to claim 1,characterized in that the boss (15) is made from an electricallyconductive material.
 8. Sensor according to claim 7, characterized inthat the boss (15) forms an electrode cooperating with thecharacterizing member (3).
 9. Sensor according to claim 8, characterizedin that a three-dimentional accelerometer is produced with the boss (3)serving as the electrode.
 10. Sensor according to claim 9, characterizedin that the second spacing (h) between the boss (15) and thecharacterizing member (3) is not in excess of a maximum authorizedtravel prior to the breaking of the characterizing member (3). 11.Sensor according to claim 1, characterized in that the boss (15) has anelectrically conductive material part, which forms an abutment surface.12. Sensor according to claim 11, characterized in that the boss (15)forms an electrode cooperating with the characterizing member (3). 13.Sensor according to claim 1, characterized in that the second spacing(h) between the boss (15) and the characterizing member (3) is not inexcess of a maximum authorized travel prior to the breaking of thecharacterizing member (3).
 14. Sensor according to claim 13,characterized in that the second spacing (h) between the cover and thecharacterizing member is between 0.5 μm and 5 μm.
 15. Sensor accordingto claim 1, characterized in that the second spacing (h) between thecover and the characterizing member is between 0.5 μm and 5 μm. 16.Sensor according to claim 1, characterized in that the cover (9) has anelectronic integrated circuit (21) electrically connected to thecharacterizing member (3) with the aid of an electrical connection (23,24, 19, 11, 19').
 17. Sensor according to claim 10, characterized inthat the electrical connection (23, 24, 19, 11, 19') comprises the ball(11).
 18. Sensor according to claim 17, characterized in that both thecover (9) and the substrate (5) has at least one interconnection element(19, 19') respectively connected to the electronic circuit (21) and thecharacterizing member (3), said elements (19, 19') being interconnectedby means of the ball.
 19. Sensor according to claim 10, characterized inthat both the cover (9) and the substrate (5) has at least oneinterconnection element (19, 19') respectively connected to theelectronic circuit (21) and the characterizing member (3), said elements(19, 19') being interconnected by means of the ball.
 20. Process for theproduction of a sensor according to claim 19, characterized in that itsuccessively comprises the following stages:producing at least one firstinterconnection element (19) on the substrate (5) alongside thecharacterizing member (3) and at least one second element (19') on aface (18) of the cover (9) turned towards the characterizing member (3),the first and second elements (19, 19') being arranged so as to face oneanother when the substrate (5) and cover (9) are integral, formationlocally on the face (18) of the cover (9) of a controlled thicknesscoating (47) constituting the boss, deposition on one of the facingelements (19, 19') of the meltable material ball, joining andinterconnecting the cover (9) and the substrate (5) by melting themeltable material.
 21. Process according to claim 20, characterized inthat the controlled thickness coating (47) is a polyimide coating. 22.Sensor according to claim 19, characterized in that the characterizingmember (3) incorporates a seismic mass (29) formed parallel to a surfaceof the substrate (5) and in accordance with a plane (x, y) containingsaid direction (y) and whereof a portion (37) constitutes a plate of acapacitor (41).
 23. Sensor according to claim 1, characterized in thatthe characterizing member (3) incorporates a seismic mass (29) formedparallel to a surface of the substrate (5) and in accordance with aplane (x,y) containing said direction (y) and whereof a portion (37)constitutes a plate of a capacitor (41).
 24. Sensor according to claim23, characterized in that the seismic mass (29) is parallelepipedic andequipped with teeth (33) able to move into grooves (35) made in thesubstrate (5).
 25. Process for the production of a sensor according toclaim 24, characterized in that it successively comprises the followingstages:producing at least one first interconnection element (19) on thesubstrate (5) alongside the characterizing member (3) and at least onesecond element (19') on a face (18) of the cover (9) turned towards thecharacterizing member (3), the first and second elements (19, 19') beingarranged so as to face one another when the substrate (5) and cover (9)are integral, formation locally on the face (18) of the cover (9) of acontrolled thickness coating (47) constituting the boss, deposition onone of the facing elements (19, 19') of the meltable material ball,joining and interconnecting the cover (9) and the substrate (5) bymelting the meltable material.
 26. Sensor according to claim 1,characterized in that the cover (9) has an electronic integrated circuit(21) electrically connected to the characterizing member (3) with theaid of an electrical connection (23, 24, 19, 11, 19').